Thin film interference filters are widely used in a variety of optical systems. Such filters are generally implemented in an optical system for reflecting one or more spectral bands of an optical signal, while transmitting others. The reflected or transmitted range, for example, may include wavelengths carrying information sensed or transmitted by the system. Failure or inadequate performance of these filters can thus be fatal to operation of a system in which they are utilized.
Interference filters are wavelength-selective by virtue of the interference effects that take place between incident and reflected waves at boundaries between materials having different indices of refraction. Typically, an interference filter includes multiple layers of two or more dielectric materials having different refractive indices. Each layer is very thin, i.e. having an optical thickness (physical thickness times the refractive index of the layer) on the order of order of ¼ wavelength of light. The layers may be deposited on one or more substrates, e.g. a glass substrate, in various configurations to provide long-wave-pass (also called long-pass), short-wave-pass (also called short-pass), band-pass, or band-rejection filter characteristics.
Conventionally, the thin film layers in very high spectral performance interference filters for use at wavelengths below about 1200 nm have been implemented using “soft coatings.” Soft coatings are typically deposited on a substrate using physical vapor deposition methods such as resistive evaporation and electron-beam evaporation. In these deposition methods, the selected coating material is vaporized, forming a cloud or stream that is imparted to the substrate. Conventional soft coating materials include metals like aluminum (Al) and silver (Ag), and dielectrics like lead fluoride (PbF2), zinc sulfide (ZnS), and cryolite (Na5Al3F14). The vaporized material solidifies on the substrate forming a thin film layer having a density and structure commensurate with the level of energy carried by the vaporized particles.
A major disadvantage associated with soft coatings is that, as the name implies, the coatings are physically soft and susceptible to damage and deterioration in most operating environments. In fact, soft coatings may be easily scratched when contacted by glass, metal, or even plastic. As such, these coatings must be protected from the environment when used in high performance applications, such as fluorescence detection systems, optical communication systems, etc. Also, because they are not very dense, they absorb moisture from the air, which causes their spectral properties to shift and can lead to longer term permanent degradation.
High performance soft coatings are, therefore, usually partially or fully hermetically sealed from the environment by placing them on the inside facing surfaces of two or more pieces of glass in a sealed ring housing, or they are sandwiched between glass substrates cemented together with optical adhesives, thus providing a barrier to moisture. FIG. 1 illustrates an exemplary prior art interference filter structure 100 including soft coating filters 102, 104 sandwiched between glass substrates 106, 108. The illustrated construction is a bandpass filter including a long-wave-pass filter 102 deposited on a first substrate 106 and affixed to the second substrate 108 via an adhesive layer 110. A short-wave-pass filter 104 is deposited on an opposing surface of the second substrate 108 and is affixed to a colored glass layer 112 by an adhesive layer 114. In addition to the effort and expense of hermetically sealing these soft coating filters, the additional substrates and optical adhesives used for such configurations lead to added loss (e.g. due to scattering and absorption) and manufacturing complexity (resulting in increased time and cost to manufacture). Another contributor to the manufacturing complexity is that in order to minimize losses associated with the additional surfaces resulting from multiple pieces of glass, additional anti-reflection (AR) coatings must be applied to these surfaces. Because of the increased cost and time required to apply additional coatings, these are often ignored; hence there is a trade-off between manufacturing complexity and filter throughput performance. Furthermore, the excess thickness associated with the hermetic seal makes it impractical for such filters to be diced into very small (e.g., millimeter-sized) filter “chips.”
In order to minimize deviation of a light beam passing through the filter construction in an imaging application, as in an optical microscope, the overall construction should have a minimal wedge angle. However, when two or more pieces of glass are cemented together, it is difficult to ensure parallelism of the interfaces and hence minimal overall wedge angle. The difficulty minimizing the wedge angle in conventional filters may cause an undesirable phenomenon known as image shift. In an optical system, such as a fluorescence imaging system as shown in FIG. 16, image shift may occur when two or more different images, each captured with a different filter set, are shifted relative to one another in the plane of the camera film, Charge Coupled Device (CCD) sensor, or other detection system, due to non-zero beam deviation caused by the emitter filters and/or dichroic beamsplitters. The exciter filter does not generally contribute appreciably to this problem. In high-performance fluorescence imaging systems (such as fluorescence microscopes) that use optical filters (e.g., emission filters, excitation filters and dichroic filters), the filters may be placed in a region of the optical system where the imaging rays are substantially collimated or parallel. Therefore, a non-zero beam deviation caused by the emitter or dichroic filters causes the image in the image plane to be displaced laterally, which is problematic. In many fluorescence experiments, it is desirable to capture multiple images of a single object marked with multiple colors of fluorescent material using a monochrome camera. Each image depicts a single color and the images are subsequently combined electronically. If there is a relative image shift among the images, the final composite image will be blurred due to the relative lateral shift of the different color components.
As shown in FIG. 17, a conventional filter based on soft coatings is comprised of multiple glass substrates bonded together with epoxy. Thus, it is difficult to control the overall parallelism of the structure and the resulting net wedge angle causes appreciable beam deviation.
Based on the above considerations, options for minimizing image, or pixel, shift have been limited. One could measure each filter and hand-select only those with the smallest beam deviation; however, the yield with this approach is low because of the difficulty controlling the beam deviation. One could rotate the orientation of the emitter filter to best offset the beam deviation of the dichroic beamsplitter; however, this approach only works well when each filter in the pair has a similar magnitude of beam deviation, and it requires the extra manufacturing steps of measuring, orienting, and marking the finished filters. One could measure the net beam deviation (and orientation) of the emitter-dichroic pair, and then add an appropriately oriented compensating wedge of the appropriate wedge angle to cancel the net beam deviation. This technique works but is expensive and time-consuming to implement and adds additional optical surfaces that can contribute to light loss in the system.
Accordingly, there is a need for filters of sufficiently low beam deviation that minimize the wedge angle and the resulting image shift and that are relatively simple to manufacture.